This invention is particularly concerned with thermoforming operations in which a web or sheet of thermoplastic material is first softened by heat and then formed or shaped while in this stretchable plastic state by being drawn or pressed against the contours of a mold or die under the influence of a fluid pressure differential. The former may be exemplified by the application of pressure within a pressure box or female mold assembly produced by compressed air or the like, to press the softened material against the contours of a male mold or core pin. Alternately, a vacuum may be applied from within a female mold while allowing atmospheric pressure to shape the heat softened thermoplastic material against the surface of the female mold. For many purposes, a combination of these two techniques is preferred with the softened sheet material being subjected to vacuum on the face contacting the mold surfaces and to a substantial positive pressure on its other face as such combination provides greater versatility and better control in the forming operation.
Apparatus for effecting thermoforming operations, in general, is comprised of a press having upper and lower platens on which are mounted a male mold assembly and female mold or pressure box assembly. In this regard, the male mold is generally mounted on the lower platen, however, this is normally a matter of choice. Prior to forming of the sheet of thermoplastic material as hereinabove discussed, a clamping assembly normally constituting a part of the male mold assembly is first caused to trap the sheet of thermoplastic material about the entrance into the female mold of the pressure box assembly including a cooperating clamping grid. Thereafter, the male mold of the male mold assembly is caused to pass through the plane of the thermoplastic material and eventually form the hollow article, as hereinabove discussed. Relative movement of the male and female mold assemblies is effected by hydraulic cylinder or electric motors assemblies together with associated cams, gears, support arms, etc., to move the platens whereas movement of a clamping assembly is effected by an associated cylinder assembly.
The hydraulic cylinder or electric motor assemblies are large even for small thermoforming apparatus and have large energy requirements. In this application the term fluid cylinder assembly is to be interpreted as the combination of a cylinder, piston and rod which is operated by a pressurized fluid i.e. a gas or a liquid.